Line pipes, which are used for transporting natural gas, crude oil, and the like, have been strongly required to have higher strength in order to improve transport efficiency by using higher pressure and improve on-site welding efficiency by using pipes with thinner walls. In particular, line pipes for transporting high-pressure gas (hereinafter also referred to as high-pressure gas line pipes) are required to have not only material properties such as strength and toughness, which are necessary for general-purpose structural steel, but also material properties related to fracture resistance, which are specific to gas line pipes.
Fracture toughness values of general-purpose structural steel indicate resistance to brittle fracture and are used as indices for making designs so as not to cause brittle fracture during use. For high-pressure gas line pipes, prevention of brittle fracture alone for avoiding catastrophic fracture is not sufficient, and prevention of ductile fracture called unstable ductile fracture is also necessary.
The unstable ductile fracture is a phenomenon where a ductile fracture propagates in a high-pressure gas line pipe in the axial direction of the pipe at a speed of 100 m/s or higher, and this phenomenon can cause catastrophic fracture across several kilometers. Thus, a Charpy impact absorbed energy value and a DWTT (Drop Weight Tear Test) value necessary for preventing unstable ductile fracture are determined from results of past gas burst tests of full-scale pipes, and high Charpy Impact absorbed energies and excellent DWTT properties have been demanded. The DWTT value as used herein refers to a fracture appearance transition temperature at which a percent ductile fracture is 85%.
In response to such a demand, Patent Literature 1 discloses a steel plate for steel pipes that has a composition that forms less ferrite in a natural cooling process after rolling, and a method for producing the steel plate. By performing the rolling at an accumulated rolling reduction ratio at 700° C. or lower of 30% or more, the steel plate has a microstructure including a developed texture and composed mainly of bainite, and the area fraction of ferrite present in prior-austenite grain boundaries is 5% or less, so that the steel plate is provided with a high Charpy impact absorbed energy and excellent DWTT properties.
Patent Literature 2 discloses a method for producing a high-strength, high-toughness steel pipe material having a composition the carbon equivalent (Ceq) of which is controlled to be 0.36 to 0.60, a high Charpy impact absorbed energy, excellent DWTT properties, and a thickness of 20 mm or more, the method including primary rolling at an accumulated rolling reduction ratio of 40% or more in a non-recrystallization temperature range, heating to a recrystallization temperature or higher, cooling to a temperature of Ar3 transformation temperature or lower and (Ar3 transformation temperature −50° C.) or higher, secondary rolling at an accumulated rolling reduction ratio of 15% or more in a two-phase temperature range, and accelerated cooling from a temperature higher than or equal to Ar1 transformation temperature to 600° C. or lower.
Patent Literature 3 discloses a method for producing a high-tensile steel plate for line pipes that has a mixed microstructure composed of 90% or more (by volume) of tempered martensite and lower bainite and has a high Charpy impact absorbed energy and excellent DWTT properties, the method including hot-rolling a steel containing, by mass %, C: 0.04% to 0.12%, Mn: 1.80% to 2.50%, Cu: 0.01% to 0.8%, Ni: 0.1% to 1.0%, Cr: 0.01% to 0.8%, Mo: 0.01% to 0.8%, Nb: 0.01% to 0.08%, V: 0.01% to 0.10%, Ti: 0.005% to 0.025%, and B: 0.0005% to 0.0030% at an accumulated rolling reduction ratio of 50% or more in an austenite non-recrystallization range, performing cooling from a temperature range higher than or equal to Ar3 transformation temperature to a temperature range of Ms temperature or lower and 300° C. or higher at a rate higher than or equal to a critical cooling rate for martensite formation, and performing on-line heating.
Patent Literature 4 discloses a method for producing a high-strength steel plate having a thickness of 15 mm or less. By rolling a steel containing, by mass %, C: 0.03% to 0.1%, Mn: 1.0% to 2.0%, Nb: 0.0.1%, to 0.1%, P≤0.01%, S≤0.003%, and O≤0.005% in a temperature range from (Ar3+80° C.) to 950° C. at an accumulated rolling reduction ratio of 50% or more, performing natural cooling for a while, and performing rolling in a temperature range from Ar3 to (Ar3−30° C.) at an accumulated rolling reduction ratio of 10% to 30%, the steel plate has an undeveloped rolling texture and deformed ferrite, undergoes no separation, and has a high absorbed energy.
Patent Literature 5 discloses a high-tensile steel plate having high toughness, excellent high-speed ductile fracture properties, and high weldability, and a method for producing the steel plate, the method including rolling a steel having a carbon equivalent, expressed by Pcm (=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B), of 0.180% to 0.220% at an accumulated rolling reduction ratio of 50% to 90% in an austenite non-recrystallization temperature range, performing cooling from a temperature higher than or equal to (Ar3−50° C.) at a cooling rate of 10° C./s to 45° C./s, stopping the cooling when the steel plate temperature reaches 300° C. to 500° C., and then performing natural cooling. In the steel plate, the fraction of Martensite-Austenite constituent in a surface portion is 10% or less, the fraction of a mixed microstructure composed of ferrite and bainite in a portion internal to the surface portion is 90% or more, the fraction of bainite in the mixed microstructure is 10% or more, the bainite includes a lath having a thickness of 1 μm or less and a length of 20 μm or less, and the lath in the bainite includes a precipitated cementite particle having a major axis of 0.5 μm or less.